Fix problem submission to work with bulk saves

The input types weren't being saved, so the checker would make a new
block with the same ids expecting to find the values in the db, but they
hadn't been saved yet.  Problem now has an implicit save after the
submit call, just as handlers and views do.

Also changed the student_input field to Any, since it can store
non-string values due to its own type conversion.
Make XBlock.__repr__ more bullet-proof

When something goes wrong in a field, it will raise an error when trying
to produce the repr of the block, which wreaks havoc with debuggers,
etc.  A try/catch makes the repr impervious to such infelicities.
Fix pep8 and pylint violations
Merge pull request #62 from edx/ned/fix-problem-submission

Ned/fix problem submission